When selecting a 13.8 V DC power supply for a transceiver, what design specification is most important?

Amateur radio transceivers draw vastly different amounts of current depending on their operating state. While receiving, a radio might draw only 1 or 2 amperes. However, when transmitting at 100 watts, the same radio can easily demand 20 amperes or more from the power supply. Because of this heavy transmit load, ensuring the power supply has a sufficient output current (amperage) capability is the single most critical design specification. If the power supply cannot deliver the required current, the voltage will drop catastrophically, which can result in distorted transmitted signals, equipment shutdowns, or damage to the power supply itself. While voltage and current metering, undervoltage protection, and convenient output connections are nice secondary features, they cannot compensate for a supply that fundamentally lacks the electrical muscle (current capability) to run your transmitter.